Physical bending of heteroepitaxial diamond grown on an Ir/MgO substrate

With increasing demand for large-diameter heteroepitaxial diamond, understanding the origin of the thermally induced bending of heteroepitaxial diamond, which is caused by the difference in the coefficient of thermal expansion between diamond and the substrate during cooling after the growth process, has become an important task. We investigated the physical bending of heteroepitaxial diamond grown on Ir/MgO substrates, and the state of the grown diamond as a result of bending was summarized as a function the diamond film thickness. When the diamond thickness (dd) was less than ∼2.5 μm, partial delamination of the diamond/Ir from the MgO substrates was observed. When dd was approximately 2.5–200 μm, diamond/Ir was delaminated and broken into small pieces. The small pieces of diamond/Ir films were highly convexified. When dd was greater than ∼200 μm, the diamond/Ir films spontaneously separated from the MgO substrates without any breakage, resulting in freestanding diamond films with the same dimension as the MgO substrates. The critical thickness to obtain freestanding heteroepitaxial diamond was found to be ∼200 μm. When dd was in the range 6–550 μm, the separated MgO substrates were convexly bent, meaning that the MgO was plastically deformed. Theoretical calculations of the substrate curvature after epitaxy were performed for the elastic bending of the diamond/Ir/substrate heteroepitaxial system. On the basis of the calculations, the maximum curvature by physical bending was expected to reach −22,800 km−1 at ∼150 μm thickness and to change to reduction with increasing diamond thickness. The curvature of breakage-free freestanding diamond samples with 243–550 μm thickness ranged from −7200 to −4400 km−1, showing moderate matching with the theoretical calculations of physical bending for the diamond/Ir/MgO multilayered system. Freestanding diamond films and MgO continued to affect each other at least until the structure experienced plastic deformation.